1. Field of the Invention
The present invention relates generally to a system for monitoring fills of liquid in a tank container and ongoing liquid condition, and particularly to a system in which a liquid quantity and other characteristics are continuously monitored and recorded for user access on site or through a remote terminal.
2. Description of the Known Art
Systems that monitor the height or volume of liquid in a storage tank, are known to be presently employed in chemical, industrial and food processing applications.
It is known from, for example, U.S. Pat. No. 4,446,730 (May 8, 1984) that the height of a liquid in a tank can be determined independently of specific gravity, by locating two pressure sensing devices in the tank at different levels. As long as the liquid level exceeds the height of the upper sensing device, the pressure difference between the sensing devices and the location of the devices in the tank, are used to compute the height of the liquid without knowledge of the liquid's specific gravity. See also U.S. Pat. No. 4,699,309 (Jun. 2, 1987), and U.S. Pat. No. 3,038,336 (Jun. 12, 1962).
The vertical separation between the pressure sensing devices must be known precisely and not allowed to deviate to ensure accuracy when computing the liquid height. Minor variations in the effective vertical separation between the sensing devices, from the value assumed in the measurement computations, will introduce significant error in the measurements. Generally, each sensing device is individually positioned inside the tank, with the lower device set a certain distance above the tank floor and the upper device positioned a predetermined vertical distance above the lower device. The mentioned '309 patent shows, in FIG. 3, a pressure sensing probe arrangement in the form of two concentrically arranged tubes with a longer tube inside a shorter tube. A spacer projects from the bottom of the longer tube to establish a known separation from the floor of the tank.
U.S. Pat. No. 4,084,435 (Apr. 18, 1978) shows a system for measuring water level at the bottom of a tank containing gasoline. The system includes two pressure sensing devices in the form of an inner air bell and an outer air bell. The air bells are arranged coaxially about one another in a manner similar to that of the probe arrangement in FIG. 3 of the '309 patent.
Knowledge of the actual quantity and condition of liquid contained in a storage tank at any given time is of course desirable, particularly if the liquid is one that is being consumed in an ongoing manner, e,g., a fuel oil, gasoline, and the like. Storage tanks from which consumable liquids are dispensed to trucks, wagons or other delivery vehicles, must be replenished regularly to ensure that a sufficient supply of the liquid will always be available. Likewise, on site storage tanks from which liquids such as fuel oil are piped to heating plants of industrial, commercial and residential buildings, need to be monitored constantly so that enough oil can be ordered and delivered in time to avoid a heating shut off. It is also important to be able to monitor and to keep accurate records of oil deliveries, including quantity of oil delivered, oil temperature, specific gravity, time of the deliveries and the like.
Tank liquid quantity measuring systems such as those disclosed in the mentioned patents, tend to be relatively expensive for commercial and most residential tank users. Those groups generally use less expensive indicators such as a dip stick having measurement scale markings. The stick is lowered through a fill opening of the tank, while holding the stick vertical until the bottom end of the stick strikes the tank floor. Liquid quantity, i.e., gallonage, is then estimated by using the height of liquid in the tank as measured on the dip stick markings, and a printed conversion table. Use of the dip stick thus requires a person physically to access the fill opening of the tank wherever it may be situated, and lower the stick through the opening while being careful to maintain the stick in vertical alignment at all times. Accuracy of the dip stick method suffers even more if the tank is not aligned horizontally, i.e., the tank is tilted such that the top surface of the liquid is not parallel to the tank floor.
Even when the commercial or residential tank user is aware that a delivery of heating oil is necessary to replenish the tank and an oil delivery is requested, he or she usually has no independent means of verifying the quantity of oil added to the tank by the supplier. If the tank has a capacity of, for example, 5,000 gallons, the exact amount of oil needed to top off the tank can be determined only if the amount of oil in the tank just prior to the delivery is first measured. Unless the tank is completely empty and the owner knows that no more than 5,000 gallons is needed to fill it, the owner must rely on the accuracy and integrity of the supplier's oil pump metering device and the honesty of the supplier's employees. With fuel oil prices fluctuating greatly and business expenses continuing to increase, the tank owner can ill afford not to be without an accurate, reliable and relatively inexpensive arrangement for continuously monitoring the quantity and quality of heating oil contained in the tank at all times.
Residential and commercial building owners who are responsible for maintaining a minimum temperature inside their buildings, often have a need to monitor the condition of the building tanks at various times from remote locations. Such remote monitoring would enable the owner to (1) determine if a fuel oil delivery is necessary so he or she can take steps to effect the delivery as soon as possible, and (2) determine accurately the quantity of fuel oil that was pumped into the tank when the delivery was made, the specific gravity of the delivered oil and its temperature.